Today: Tesla's sharp downturn flips to a gain for a day as CEO Elon Musk defends company amid a federal investigation into Model S fires. Also: Apple (AAPL)-Samsung trial draws to a close, solar companies decline.

The Lead: Tesla faces federal investigation, CEO promises success

A tough period in the rapid rise of Tesla Motors (TSLA) seemed to hit its nadir Tuesday, as a federal agency announced an investigation into recent fires involving the first electric car the Palo Alto company fully designed and manufactured, but Tesla's declining stock turned around as CEO Elon Musk exuded confidence amid the maelstrom.

The NHTSA is most concerned with any possible Model S vulnerability to fires as a result of objects hitting the underside of the car, stating in a report that the investigation's purpose is "to examine the potential risks associated with undercarriage strikes."

Musk characterized the media coverage of the fires as unfair, pointing out that fires involving gasoline-powered cars have caused more than 400 deaths and roughly 1,200 injuries in the past year, but the three noninjury fires in Tesla automobiles received far more attention than all of those combined.

"Reading the headlines, it is therefore easy to assume that the Tesla Model S and perhaps electric cars in general have a greater propensity to catch fire than gasoline cars when nothing could be further from the truth," Musk wrote, later adding, "the Model S is safer in an accident than any other vehicle without exception."

Musk went on to say that Tesla had taken three steps to ensure confidence in its automobiles, including requesting the NHTSA investigation, though a spokesman for the agency said he was unaware of such a request. Tesla also issued a software update to its cars that will keep the underside of the car farther away from the road on the highway and amended their warranty policy to provide coverage for any fire, including blazes caused by driver error.

Other Silicon Valley stocks gaining Tuesday included Broadcom, which led the SV150 with an 8.5 percent gain to $8.78 after announcing earnings that beat Wall Street's expectations. Social-networking stocks enjoyed a bountiful day as well, with Facebook rising 1.2 percent to $46.36, Zynga gaining 3.9 percent to $3.99, and Twitter adding 1.5 percent to $41.75 to bounce back from Monday's weakness; LinkedIn was the outlier, with a 0.3 percent decrease to $221.38. Intel (INTC) gained 0.4 percent to $24.70 ahead of its analyst day Thursday, and eBay (EBAY) gained 0.1 percent to $52.12 after PayPal struck a deal to handle payments for car-sharing company Uber.

Silicon Valley's solar companies suffered Tuesday, as Chinese solar companies showed they may be rebounding from weakness that had boosted their American counterparts and increased prices for solar panels. SunPower (SPWRA) fell 6.6 percent to $30.69 and SolarCity declined 8 percent to $46.72 after Trina Solar became the third Chinese solar company to report a profitable quarter after a backlash against the firms following their overproduction of cheap solar panels, which caused a glut of the material nationwide.

And the widely watched Standard & Poor's 500 index: Down 3.66, or 0.2 percent, to 1,787.87

Check in weekday afternoons for the 60-Second Business Break, a summary of news from Mercury News staff writers, The Associated Press, Bloomberg News and other wire services. Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/jowens510.